This invention relates to a wideband solid-state analog switch and more particularly to such a switch for use in an M-by-N analog crosspoint matrix switch for routing one of N inputs over one or more of M outputs.
Multichannel mobile and satellite communications systems require switch arrays that can route high frequency signals to various points under system control. For example, a crosspoint matrix uses an M-by-N array of switches to direct any of N inputs to any of M outputs. In the past, where large input voltages are used, the switches were built from coaxial relays. However, arrays of coaxial relays were generally large, expensive and consumed a great deal of power. Therefore, it became important to build switches from solid-state switching elements, ideally all on the same semiconducting substrate.
Recent advances in semiconductor fabrication processes have made possible the application of analog switches using solid-state switching elements. Solid-state analog switches can now be made with lower on-resistance, faster switching and reduced power consumption. With improvements in MOS technology one achieves improved breakdown-voltage tolerance and reduced leakage current such that these switches can now handle signals with a range of .+-.70 V. When used in high-speed switching systems, the analog switch must possess the following characteristics:
1. Wide Bandwidth--because the number of channels in communications links is limited and signals are multiplexed at high frequencies, the switch must operate over a wide bandwidth;
2. Wide Dynamic Range With Low Distortion--the switch must pass the signal with little distortion over the dynamic range of the signal (non-linearities in the input-output transfer characteristic of the switch lead to differential-gain error in analog signals);
3. High Isolation--to avoid crosstalk between channels, the switch, when open, must not allow an input signal to leak through to an output;
4. Low Insertion Loss--the switch must be transparent (offer low resistance) to the input signal when it is closed; and
5. Broadcasting Capability--the switch must be able to route one input signal to more than one output. It is understood that for the signal not to be degraded in this mode, the loading of the switch elements on the input signal must be small.
Presently, T-switches are employed in these high-speed switching systems. The T-switch employs three N-channel field-effect transistors (FETs). The arms of the T are formed by two FETs connected in series along the drain-source path. They act as pass-transistors and when turned on, the first FET passes the input to the second FET which outputs the signal. The third FET is connected from ground to the point between the two pass transistors. When the first and second transistors are turned off, the third transistor is turned on; therefore, any AC current that leaks through the first pass-transistor is shunted to ground. In this manner, the third FET prevents leakage current from being output. When CMOS technology is used, the N-channel FETs are replaced by transmission-gates employing paralleled N-channel and P-channel devices.
The T-switch is fabricated on a single semiconducting substrate. While it offers good isolation, small size and low power dissipation, all at a low cost, the T-switch has one major drawback: the aforementioned advantages are only available when the T-switch operates within a limited dynamic range over a wide dynamic range, the linearity of the T-switch is poor because the on-resistance of the two pass-transistors varies with the signal level. In addition, the bandwidth is limited by the series resistance of the two pass-transistors. The series resistance also causes insertion loss at low frequencies, thereby attenuating the signal voltage whenever the source resistance load resistance are low. Because the loading of the transmission line feeding the switch typically ranges between 50 and 75 ohms, the series resistance of the pass-transistors (40-100 ohms) introduces a 5-10 dB loss. Although the resistance can be lowered by increasing the size of each pass-transistor, the resulting increase in shunt capacitance still degrades the bandwidth. By compensating for this loss with subsequent amplification of the output signal, the bandwidth is further limited.
Furthermore, broadcasting does not work well with the T-switch. The high loading of the transmission line feeding the switch degrades the output signal as each switching point that is turned on introduces additional loading on the input signal, thereby further increasing the attenuation. The transmission line cannot be properly terminated in its characteristic impedance unless a resistive termination is placed before the switch, and the switch is followed by a buffer or amplifier to drive the output cable.
The Siliconix DG536 is an example of an off-the-shelf, high-performance wideband analog switch. Its bandwidth is 300 MHz and its all-hostile crosstalk level is -60 dB at 5 MHz.
The prior art was aware of the need to provide a wideband analog switch. Reference is made to the following:
An article entitled "Improved Analog Switches & Multiplexers Bring Benefits To Old & New Applications" by Peter Harold in EDN, May 14, 1987, pp. 65-74. The article gives examples of CMOS circuits employed as switches available from different manufacturers with the operating characteristics. The article discusses the term "All-Hostile" cross talk.
See article entitled "Gigahertz-Band Analogue Switch Using Bipolar Super Self-Aligned Process Technology" by H. Kikuchi et al., Electronic Letters, Sept. 12, 1985, Volume 21, No. 19, pages 854-855. The article describes a bipolar analog switch using super self-aligned process technology (SST). The switch employs three emitter-coupled pairs and achieves a net isolation of 40 dB and low third-order intermodulation of less than -40 dB below the input level of -7 dBm at 1 GHZ. The schematic of the switch is shown in FIG. 1 in the article.
See also an article entitled "A GaAs Switching IC for a Gigabits Per Second Communication System" by Y. Nakayama-et al., published in the IEEE Journal of Solid State Circuits, Vol. SC-21, No. 1, February, 1986, pp. 157-161. The articles describes a monolithic 4.times.4 switching circuit which employs built in address decoders and is ECL compatible.
Therefore, it is an object of the present invention to provide an improved analog solid-state switch which operates over a wide bandwidth while offering little distortion of the input signal.
An additional object of the present invention is to implement the improved analog solid-state switch with field-effect transistor (FET) technology.
A further object of the present invention is to increase the dynamic range of the analog solid-state switch by matching the non-linear transfer characteristics of the FETs.
Yet a further object of the present invention is to reduce insertion loss of the input signal by matching the non-linear transfer characteristics of the FETs.
A further object of the present invention is to implement the wideband analog solid-state switch in an M-by-N crosspoint matrix switch for switching one of N inputs across one or more of M outputs.